Why is war in decline? For one thing, it no longer pays. For centuries, wars reallocated huge territories, as empires were agglomerated or dismantled and states wiped off the map. But since shortly after World War II, virtually no borders have changed by force, and no member of the United Nations has disappeared through conquest. The Korean War caused a million battle deaths, but the border ended up where it started. The Iran-Iraq War killed 650,000 with the same result. Iraq’s annexation of Kuwait in 1990 backfired. Israel seized land in 1967, but since then most has been returned and the rest remains contested…

War also declines as prosperity and trade rise. Historically, wealth came from land and conquest was profitable. Today, wealth comes from trade, and war only hurts. When leaders’ power depends on delivering economic growth, and when a country’s government becomes richer and stronger than its warlords, war loses its appeal.

Perhaps the deepest cause of the waning of war is a growing repugnance toward institutionalized violence. Brutal customs that were commonplace for millennia have been largely abolished: cannibalism, human sacrifice, heretic-burning, chattel slavery, punitive mutilation, sadistic executions. Could war really be going the way of slave auctions? Nothing in our nature rules it out. True, we still harbor demons like greed, dominance, revenge and self-deception. But we also have faculties that inhibit them, like self-control, empathy, reason and a sense of fairness. We will always have the capacity to kill one another in large numbers, but with effort we can safeguard the norms and institutions that have made war increasingly repugnant.